1. Field of the Invention
The present invention relates to humidity control and especially to sensing the amount of moisture contained in heated exhaust air in laundry dryers to permit control of the dryer.
2. Description of the Prior Art
Generally, it has been recognized that it is desirable to sense the moisture content of clothes or other laundry articles during drying operations. Clothes may be taken out of the dryer prior to the moisture level being sufficiently low for them to be dry or they may be left in a dryer too long, resulting in excess drying, which causes scorching and is wasteful of energy. Thus, the prior art has suggested methods of detecting the moisture remaining in the articles being dried. Conventionally, these are placed within the drying chamber and the articles are tumbled about in close proximity to the sensors. This type of system, however, presents a significant problem in that the surfaces of the humidity sensor quickly become corroded and fouled with lint and other material. Thus, they must be cleaned frequently and are apt to provide false readings. Helfrich in U.S. Pat. No. 3,436,838 and Beller in U.S. Pat. No. 3,521,376 disclose exemplary clothes drying systems.
Numerous other systems have also been suggested in the art that utilize moisture sensors. For example, Anderson U.S. Pat. No. 2,407,215 and Sobkow U.S. Pat. No. 3,555,289 both disclose automobile windshield wiper systems actuated by moisture deposited between electrodes located in the wiper path. In another system, Trischberger U.S. Pat. No. 3,467,860 discloses a method of testing the moisture content in a solid.
Numerous other systems and electrode configurations for humidity sensors are disclosed in the art. For instance, see Auwarter U.S. Pat. No. 2,715,667, Miller U.S. Pat. No. 2,733,607, Seehof et al U.S. Pat. No. 2,904,765 and Weston et al U.S. Pat. No. 2,930,016. In each of these cases, the electrodes of the sensors are narrowly separated by a solid humidity-sensitive conductor. Also, it has been suggested that humidity changes be determined by sensing capacitance changes utilizing a narrow insulated gap between two electrodes. This type of system is embodied in Misevich et al U.S. Pat. No. 3,350,941 and Krobath U.S. Pat. No. 3,424,977. Lastly, the use of widely spaced electrodes has been suggested in Sion U.S. Pat. No. 2,819,614 to measure resistivity changes in a thermistor heated by current flow to an adjoining electrode.
Thus, it is clear that there is a need in the art for a moisture detecting system which is simple and low cost and still be accurate in its operation. The need is also apparent for a system that will not interfere with the drying of clothing or laundered materials, and which will accurately indicate when the articles are sufficiently dry. It is to these ends that the present invention is directed.